The present invention is a platinum curing organosiloxane composition having improved hysteresis characteristics when cured. The present compositions are particularly useful as materials of fabrication for keyboard pads used in electronic keyboards.
The use of various platinum cured organosiloxane compositions as material of construction for keyboard pads is well known in the art. A common problem associated with such previously known organosiloxane compositions is poor hysteresis characteristics, evidenced as a lack of rebound or crispness in the feel of the keys of a keyboard when depressed. The present inventors have discovered a unique combination of vinyl-substituted organosiloxane polymers, filler concentration, filler treating agents, and organohydrogensiloxane resin crosslinking agent having a high level of hydrogen substituted on silicon, that provides a cured organosiloxane composition having excellent hysteresis characteristics.
Jensen, U.S. Pat. No. 4,753,978, teaches an organosiloxane composition comprising two diorganovinylsiloxy terminated polydiorganosiloxanes which can be platinum cured to form elastomers having high tear strength. Jensen teaches a curable mixture comprising 70 to 95 percent by weight of the mixture of a diorganovinylsiloxy terminated polydiorganosiloxane containing vinyl or other ethylenically unsaturated hydrocarbon radicals only at the terminal positions and a second polydiorganosiloxane containing both terminal vinyl radicals and from 1 to 5 mole percent of vinyl radicals on non-terminal repeating units. Jensen teaches the composition can contain 10 to 60 weight percent, based on the weight of the composition, of a reinforcing silica filler and that the filler can be treated with a treating agent such as a hexaorganodisilazane. Jensen also teaches the use of a dimethylsiloxane/methylhydrogensiloxane copolymer as a crosslinker, where the copolymer comprises from 10 to about 50 repeating units per molecule and from 3 to 5 of the repeating units are methylhydrogensiloxane.